Inflatable reflector antennas offer low-mass, low-volume options when high-gain antennas are required. While the antennas are inflating from their packaged state, their geometrical properties change over the inflation time. It is desirable to characterize the radiofrequency performance of the antennas during inflation so that the solar effects can be emulated from the coefficient of thermal expansion and solar flux variation experienced by the antenna in the space environment, where the changes occur on time scales on the order of minutes. Changes in the solar effects will impact antenna performance over the lifetime of the antenna. These solar effects can only be emulated within a thermal-vacuum chamber. Although near-field and/or far-field laboratory equipment is not available in the vacuum chamber in which inflation takes place, antenna surface characteristics can be obtained by laser-ranging the antenna surface with mirrors from the outside of the vacuum chamber.
The Reflector Antenna Surface Photogrammetry Study is the first part of an effort at the NASA Glenn Research Center to characterize the radiofrequency secondary patterns of an inflatable antenna. Initial laser scans in combination with near-field antenna patterns of the antenna surface show that the commonly used Ruze theory, which estimates gain degradation from the wavelength and the root-mean-square surface error overstates the loss in gain for inflatable antennas.
The Ruze theory assumes that the surface errors are random gaussian-distributed errors. However, inflatable antennas experience non-gaussian-distributed surface errors that are correlated, for example, with a wrinkle. Ideally, reflector antennas have reflections that are in the direction that the antenna is pointing. However, when surface errors are introduced, the direction of the reflections can vary depending on the location of the surface error and the geometry of the error in comparison to the ideal antenna surface.
The next phase in analyzing the secondary patterns of the inflatable antennas is to examine the laser photogrammetry data and to compute not only the correct gain (and therefore the correct degradation from the ideal antenna surface), but also the secondary pattern. The second phase of the planned analysis is not accounted for by the Ruze theory because the theory assumes random gaussian-distributed surface errors. The goal is to obtain the secondary patterns and the antenna gain from the photogrammetry data, such that vacuum chamber tests can be performed while photogrammetry data are obtained to characterize how the inflation of the inflatable antenna changes the antenna performance over time.
Left: Reflection from ideal parabolic surface. Right: Reflection from corrupted parabolic surface.
Long description of figures 1 and 2.
Photogrammetry data of antenna surface.
Long description of figure 3.
The Reflector Antenna Surface Photogrammetry Study effort is managed under the Space Communications and Data Systems Project at Glenn. The work was performed in-house by a member of the Communications Systems Integration Branch in Glenn’s Communications Technology Division.
Find out more about the research of Glenn’s Communications Technology Division: http://ctd.grc.nasa.gov
Glenn contact:
Bryan W. Welch, 216-433-3390, Bryan.W.Welch@nasa.gov
Author:
Bryan W. Welch
Headquarters program office:
Space Communications Technology Program
Programs/projects:
Space Communications and Data Systems Project
Last updated: December 14, 2007
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